Responses of soil gross nitrogen transformations to three vegetation restoration strategies in a subtropical karst region
Dejun Li, Li Wen, Kongcao Xiao, Tongqing Song, Kelin Wang
Abstract
Abstract Investigation of soil internal nitrogen cycles is useful for unravelling the mechanisms responsible for soil nitrogen availability change. Here, gross nitrogen transformations were investigated in a subtropical karst area after 16 years of vegetation restoration. The experiment included four treatments with one control and three restoration strategies, that is: (i) maize‐soybean rotation field (CR, control); (ii) restoration with Toona sinensis (TS); (iii) restoration with Guimu‐1 hybrid elephant grass (GG, Pennisetum americanum (L.) Leeke × Pennisetum purpureum Schumach); and (iv) restoration with Guimu‐1 hybrid elephant grass and Zenia insignis as a mixture (ZG). Soil NH 4 + and NO 3 − responded differentially to three restoration strategies, resulting in no significant change of total inorganic N content following vegetation restoration. Gross nitrogen mineralization (GNM) rate increased by 124.0%, 96.8%, and 60.3% in TS, GG, and ZG, respectively, relative to CR (2.88 ± 0.05 mg N kg −1 d −1 ). The strongest explanatory variable was microbial biomass carbon for both GNM ( R 2 = 0.82) and gross nitrification ( R 2 = 0.66). Dissimilatory NO 3 − reduction to NH 4 + (DNRA) rate in TS or ZG, which was significantly greater than that in CR, was significantly correlated with SOC: NO 3 − ratio ( R 2 = 0.61), implying that DNRA was limited by SOC availability. Gross NH 4 + immobilization rate, which was highest in GG, and lowest in CR, was best predicted by carbon to nitrogen ratio ( R 2 = 0.65). However, gross NO 3 − immobilization rate could not be well predicted by the measured soil properties. Our results suggest that vegetation restoration especially TS significantly enhanced soil N availability as reflected by gross N mineralization rate.